ORA-02298 error

ORA-02298: cannot validate (A.TE_ID ) - parent keys not found

solution:

select xxxxx from child_table a where not exists (select 'x' from
parent_table where primary_key = a.key_values);

This will show you the row (or rows) that exists in the child table that
don't exist in the parent table. Once you have either (added rows
to the parent table to match the child rows) or (removed the child
rows that don't have parents), then you can enable your RI constraints

SQL> select count(te_id)
2 from A a
3 where not exists (select 1 from B tp where tp.te_id = a.te_id);

COUNT(TE_ID)
------------
15362

SQL>
SQL> delete from a
2 where not exists (select 1 from B tp where tp.te_id = a.te_id);

19573 rows deleted

SQL>
SQL> select count(te_id)
2 --delete from a
3 where not exists (select 1 from B tp where tp.te_id = a.te_id);

SQL>
SQL> select count(te_id)
2 --delete
3 from a
4 where not exists (select 1 from B tp where tp.te_id = a.te_id);

COUNT(TE_ID)--DELETE
--------------------
0

SQL> commit;

Commit complete

SQL> alter table a add constraint A_B_FK foreign key (TE_ID) references B (TE_ID);

Table altered

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